This invention relates to a gas flow control system for use in a low temperature oxide chemical vapor deposition system. More particularly, the invention relates to a system in which mass flow controllers are used to control the flow of gases through a plurality of gas injectors positioned within the reaction tube of a vacuum low temperature oxide chemical vapor deposition system.
In the fabrication of semiconductor devices, it is desirable to deposit layers of silicon dioxide, which is a dense, chemically inert dielectric material having extreme hardness, low thermal conductivity and high resistance to molecular diffusion. Silicon dioxide is used, not only as an intermediate dielectric layer of multilayer metal devices, but it is also used in connection with doped silicon dioxide.
It has, in the past, been found to be difficult to deposit silicon dioxide onto semiconductor substrates in a manner which will produce uniform layers having a high quality free of defects in an economical manner. The art has developed from the use of one-atmosphere systems utilizing so-called cold wall furnaces to low pressure or partial vacuum deposition systems in which the silicon dioxide is deposited by oxidizing silane on the surfaces of the semiconductor substrates, which are heated inside an evacuated chamber. In the case of deposited, doped silicon dioxide layers, gases, such as phosphine, are mixed with silane to deposit a doped silicon dioxide. The heating of the substrates prior to deposition is accomplished in a standard diffusion furnace in which the diffusion tube is provided with means for evacuating the gases contained therein prior to the introduction of the silane, phosphine and oxygen. Although the low pressure deposition provides substantial advantages over the one-atmosphere process, there are substantial difficulties in obtaining the desired uniformity of deposition, as well as the freedom from surface defects. The prior art patent to Alberti et al., U.S. Pat. No. 4,098,923, discloses a low pressure silicon dioxide deposition system, which includes a number of features to improve the uniformity of the layer deposited on the various substrates positioned vertically down the processing tube, as well as across the wafer uniformly on the substrates. The various means utilized in the Alberti et al. patent to achieve this improved uniformity involve the use of a shroud in connection with the wafer supporting boat to improve the distribution of the mixed gases across the substrates supported in the boat. In addition, the mixed gases, such as silane and phosphine and the oxygen to react therewith, were delivered not at the end of the process tube as in the prior art, but through injectors extending lengthwise of the process tube adjacent the substrate boat. These injectors were provided with openings, again positioned adjacent the substrate supporting boat, to deliver the various gases directly to the deposition area. Also disclosed in the Alberti et al. patent were valves and flow control means so that the flows and the proportions of the various gases could be controlled.
Subsequent to the filing of the Alberti et al. application, there was developed a further modification of the gas injector structure in which a plurality of separately controlled gas injectors were provided in order that the volume of mixed gas delivered in the various areas lengthwise of the tube could be altered or adjusted. In one commercial embodiment, there were five separate injectors, each having a plurality of holes to deliver a controlled flow of mixed gas to a lengthwise segment of the process tube. The rate of flow and distribution of the oxygen being less critical, there were only two injector tubes having openings to supply oxygen throughout the length of the processing zone. Although the commercial embodiments of the above described low pressure chemical vapor deposition system were equipped with flow meters and manually adjustable valves to control the rates of flow to the various gas injectors, it was known generally in the art of chemical vapor deposition systems to provide individual mass flow controllers for each of the gas injectors.